1. Field of the Invention
The present invention relates to a method for cleaning molten metal by removing inclusions suspended in molten metal and an apparatus therefor. In one aspect it involves making nonmetallic inclusions rise to the surface of the molten metal for facilitating their removal. In another aspect, the invention involves a method for refining molten steel in a vacuum, in particular a method of degassing steel.
2. Description of the Prior Art
There have been proposed various methods for decreasing or removing inclusions (for example alumina inclusions) suspended in molten metal since those inclusions can be a cause of defects and lower the quality of a product. Four proposed methods are described below:
(a) A first method, wherein inclusions in molten metal are trapped by gas bubbles produced by bubbling inert gas in molten metal at atmospheric pressure from the bottom of a vessel having molten metal therein. The inclusions are removed from the molten metal after the inclusions have risen to the surface of the molten metal.
(b) A second method, wherein inclusions in molten metal are removed by a filter made from calcium oxide which is put in a flow of the molten metal.
(c) A third method, wherein inclusions in molten metal are removed by throwing a solid such as calcium oxide capable of adsorbing the inclusions into the molten metal.
(d) A fourth method, wherein inclusions in molten metal are removed by having the inclusions rise to the surface of the molten metal or sink because of differences in densities of the inclusions and the molten metal.
However, in case of having an object of manufacturing quality material, a total amount of oxygen in molten metal needs to be limited to 15 ppm or less. When molten metal is cleaned by use of said methods, there can occur the following problems:
In the first method, a zone of the bubbling only spreads upwardly from a gas blowing-in inlet positioned at the bottom of a vessel. Moreover, there is a problem that it is difficult to bubble the molten metal from the whole vessel. When the bubbles produced by bubbling are large, the molten metal flows, bypassing the bubbles, during the bubbles' rising to the surface of the molten metal. In this case, micro-inclusions in the molten metal are hard to be trapped by the bubbles since the micro-inclusions also move together with the flow of the molten metal, bypassing the bubbles.
In the second method, when a filter capable of removing micro-inclusions is used, the filter often is choked and unable to be used soon after it has begun to be used.
In the third method, when the effectiveness of removing inclusions in molten metal by use of a solid such as calcium oxide decreases, there occurs a necessity for withdrawing the solid out of the molten metal. In this case, there is a problem that the withdrawal of the solid requires troublesome work and, moreover, the efficiency of the withdrawal of the solid is low.
In the fourth method, due to small particles of the micro-inclusions, the rising or the sinking of the micro-inclusions takes a lot of time. This leads to a decrease of the efficiency of removing the micro-inclusions.
A large amount of gas components is contained in molten steel produced in a steel-making furnace such as a converter and the like which smelts and refines steel. To remove the gas components, there are carried out various vacuum processing methods wherein molten steel is degassed in a vacuum. Out of those methods, for example, a RH vacuum degassing method is known. In this RH vacuum degassing method, a ladle is filled up with molten steel to be processed. Two immersion nozzles arranged at the lower portion of a vacuum vessel are immersed in the molten steel from the upper side of the ladle. Inert gas is blown from the middle of one immersion nozzle to have the molten steel in the ladle circulated through the immersion nozzles inside the vacuum vessel. In this way, the molten steel is degassed in the vacuum vessel.
Requirements for components of steel for a special use, however, are more severe than those of molten steel processed with the RH vacuum degassing method. Therefore, it is necessary to use other methods so as to process molten steel for a special use. To remove alumina inclusions in molten steel, for example, the total amount of oxygen in the molten steel needs to be decreased. The total amount of oxygen in the molten steel can barely be decreased to approximately 10 ppm by use of the RH vacuum degassing method. Therefore, the RH vacuum degassing method cannot be applied to steel which requires a total amount of oxygen of less than 10 ppm.